1. Field of the Invention
The field of the invention relates to optical communications systems, and more particularly, to an evanescent field coupler.
2. Related Art
Evanescent field coupling occurs when two single-mode fibers, for example, are placed parallel to one another and the distance between the core of the fibers is reduced until the evanescent fields of the two guides overlap. The fundamental mode on one fiber interacts with the fundamental mode of the neighboring fiber and power is coupled between the two fiber cores. The strength of the coupling is determined by the separation of the fiber cores, the extent to which the evanescent field spreads into the cladding, and the length of the coupling region.
A cross-sectional view of a conventional evanescent field coupler (EFC) is shown in FIG. 1. Two optical fibers 102 and 104 are mounted in substrate blocks 106 and 108, respectively, such that the cores 110 and 112, respectively, of the two fibers are in close proximity to each other permitting coupling as described above. Each fiber/substrate block is referred to as a "half" of the coupler.
Two common EFC's are shown in FIGS. 1 and 2. FIG. 1 shows a cross section of a laterally switched coupler 100. According to this technique, coupling is switched on and off by lateral separation of the fiber cores. FIG. 2 shows a cross section of a vertically switched coupler 200. According to this technique, coupling is switched on and off by vertical separation of the fiber cores. See M. Digonnet et al., "Analysis of a Tunable Single Mode Optical Fiber Coupler," IEEE of Quantum Electronics, Vol. QE-18, No. 4, pp. 746-751 (April 1982), and H. Berthou et al., "Switching Characteristics of a Piezoelectrical Actuated Evanescent-Wave Directional Coupler," in Electronic Letters, Vol. 23, No. 9, pp. 469-471 (Apr. 23, 1987), for examples of a sliding EFC and a vertical switching EFC, respectively.
EFC's, such as the one exemplified in FIG. 1, are typically made by gluing each fiber in a grooved glass substrate block, or a material of like hardness. FIG. 3 shows a cross section of such a mounting arrangement. A mounted fiber 302 and its corresponding substrate block (hereafter called the "block") 304 are ground to remove a predetermined amount of outer cladding material for a desired amount of evanescent coupling when properly aligned with another coupler half.
Evanescent field coupling theory is well understood (see, for example, articles by: A. Das et al., "Automatic Determination of the Remaining Cladding Thickness of a Single-Mode Fiber Half-Coupler," Optics Letters, Vol. 19, No. 6, pp. 384-386 (Mar. 15, 1994); A. Das & M. Pandit, "Analysis and Modeling of Low-Loss Fused Fiber Couplers," SPIE, Vol. 1365; "Components of Fiber Optic Applications," Vol. pp. 74-85 (1990); and O. Leminger et al., "Determination of Single-Mode Fiber Coupler Design Parameters from Loss Measurements," Journal of Light Wave Technology, Vol. LT-3, No. 4, pp. 864-867 (August 1985)). Polish depth, fiber bend radii, index of refraction of the fibers, refractive index of coupling liquid glue and block material hardness, index profiles of the fiber cores, and the like, are parameters that affect performance of an EFC. Conventional manufacturing techniques, however, have several drawbacks.
To date, couplers are limited to original designs. That is to say, EFC-type couplers can not be added to an optical communications system in the field during operation. Entire subsystems and/or switching networks must be shut down for new EFC's switches to be added.
Another drawback of conventional couplers/switches is their performance. Switching repeatability is poor, maintaining a coupling ratio requires frequent adjustment and fiber types need to be closely matched. Additionally, for lateral switching couplers, scratching during lateral movement can reduce the lifetime of the coupler by inducing failure through static fatigue. High optical loss during manufacture is still another disadvantage of conventional EFC devices.